The present invention relates to an optical waveplate for use in an optical communication system, a method of manufacturing the optical waveplate, and a waveguide device using the optical waveplate.
Recently, various methods have been proposed for transmission of a large quantity of information stably and inexpensively. An optical communication system is one of these methods. A representative example of this optical communication system is a method (wavelength division multiplexing system) in which light components having a plurality of wavelengths and carrying their respective signals are multiplexed into single light by a multiplexer and transmitted to a remote place through an optical fiber. The light is demultiplexed into the light components with their original wavelengths by a demultiplexer upon reception, thereby detecting the individual signals. This method can increase the communication capacity in proportion to the multiplexing number of wavelengths and is therefore a very effective method of increasing the capacity. This method also can reduce the load on hardware in an optical communication network connecting a number of points and makes a more advanced network arrangement possible by using a combination of a plurality of light wavelengths, multiplexers, and demultiplexers in addition to spatial wiring.
A system of the above sort requires a light source which oscillates at a plurality of wavelengths, and a multi/demultiplexer for multiplexing/demultiplexing light. As the multi/demultiplexer, a device using PLCs (Planar Lightwave Circuits) consisting of optical waveguides formed on a substrate has been developed as the most realistic device from the point of view of a small size, a light weight, and a high reliability. Of these PLCs, a silica-based PLC fabricated by depositing a silica glass film on a silicon substrate is expected as a practical optical component, since it has a small optical loss and consequently a high stability against disturbance such as heat or vibrations.
The most serious problem in putting the silica-based PLC into practical use is its polarization dependence. That is, as mentioned above, this silica-based PLC is manufactured by depositing a glass film on a silicon substrate. Therefore, the difference in thermal expansion coefficient between the glass film and the silicon substrate makes a stress applied on an optical waveguide in a direction parallel to the surface of the substrate differ from that in a direction perpendicular to the substrate surface. Consequently, the refractive index of the optical waveguide in the direction parallel to the surface of the silicon substrate becomes different from that in the direction perpendicular to the substrate surface. This is termed "waveguide birefringence". When, for example, an asymmetrical Mach-Zender interferometer is constituted by the silica-based PLC, this waveguide birefringence gives rise to a problem that the optical path length difference (a difference in refractive index x physical length) of an arm constituting the interferometer changes depending on the polarizing direction of light. Consequently, the device characteristics change in accordance with the polarized state of light. This makes it impossible to apply the device to a system using a single-mode fiber.
This problem of the polarization dependence of the PLC caused by the waveguide birefringence is not inherent in a silica-based glass waveguide. That is, all waveguides currently manufactured have this problem because they also have waveguide birefringence, although the degrees of waveguide birefringence differ from one another. Examples of the waveguides are a titanium in-diffused LiNbO.sub.3 waveguide, a proton-exchanged LiNbO.sub.3 waveguide, an ion-exchanged glass waveguide, a semiconductor waveguide, a polycarbonate waveguide, a polyimide waveguide, a silicone resin waveguide, and an epoxy resin waveguide.
As a method for compensating for the birefringence of a silica-based optical waveguide, a method of mounting amorphous silicon on top of a waveguide and using the resultant stress is known. This method, however, requires some additional steps, such as a step of mounting amorphous silicon and a step of trimming the amorphous silicon by using a laser, after a sample is formed, in order to finely adjust the stress. In addition, since it is difficult to compensate for the waveguide birefringence across a wide area, individual waveguides must be spaced apart from one another. It is, therefore, impossible to apply this method to waveguides integrated at a high density. As described above, the method using the stress of amorphous silicon has several practical problems.
Takahashi et al., on the other hand, have developed a method of eliminating the polarization dependence of the PLC by inserting a half waveplate consisting of a rock crystal at the center of an optical circuit of an arrayed-waveguide grating-type wavelength multi/demultiplexer such that the optical principal axis of the half waveplate forms an angle of 45.degree. with a substrate. (Hiroshi Takahashi et al., "Optics Letters," Vol. 17, No. 7, pp. 499-501 (1992)). Takahashi et al. have also pointed out in Japanese Patent Prepublication No. 4-241304 that this method is also effective in eliminating the polarization dependence of a Mach-Zender interferometer, a ring resonator, a directional coupler, and a phase modulator. This method of eliminating the polarization dependence of an optical circuit by inserting a rock-crystal half waveplate at the center of the optical circuit realizes a high reliability for long periods of time, has simple manufacturing steps, and can be applied to all waveguides in addition to a silica-based glass waveguide. Therefore, the method is very effective compared to the above-mentioned method by which amorphous silicon is mounted.
A rock crystal has a high heat resistance, a high humidity resistance, and a high precision processability and shows stable optical characteristics. Therefore, a PLC incorporating a rock-crystal half waveplate has a high reliability. However, this method has a large drawback; that is, since there is no light-confining structure in the half waveplate and in a groove for receiving the half waveplate, light propagating through the waveguide is radiated from these portions, resulting in loss of light. According to the report by Takahashi et al., an excess loss of 5 dB is produced when a half waveplate consisting of a rock crystal is inserted into a 100-.mu.m wide groove formed in a waveguide with a specific refractive index difference of 0.75%. This value is extremely large compared to a loss of 2 to 3 dB of the PLC itself. Consequently, it has been impossible to apply the method to actual PLCs from the point of view of the optical loss.
To obtain a PLC incorporating an optical waveplate as a highly practical component, it is important to decrease the excess loss produced by insertion of the waveplate to 0.5 dB or less (i.e., to reduce the decrease in the light quantity to 10% or less).
FIG. 1 shows the result of simulation of the excess loss performed by assuming that a light beam emitted from the end face of an optical waveguide is a Gaussian beam. This characteristic curve illustrated in FIG. 1 shows that the excess loss is reduced to 0.3 dB or less when the film thickness of an optical waveplate is 20 .mu.m or less.
In a practical case, however, a loss of about 0.1 to 0.2 dB is unavoidable because of Fresnel reflection or scattering at the end face of a waveplate. When this fact is taken into consideration, therefore, the film thickness of an optical waveplate must be 20 .mu.m or smaller in order to reduce the excess loss as a result of insertion of a waveplate to 0.5 dB or less. To manufacture a half waveplate, with a wavelength (1.3 .mu.m, 1.55 .mu.m) currently used in long-distance optical communication, to have a film thickness of 20 .mu.m or smaller, the material of the waveplate is required to have an in-plane birefringence greater than at least 0.03. A rock-crystal half waveplate brings about a large excess loss as described above because its thickness is as large as 91 .mu.m. This large thickness results from a small birefringence of a rock crystal of 0.0085 at a wavelength of 1.3 .mu.m. The use of a material having a large birefringence makes it possible to manufacture a thin waveplate, and this results in a decreased excess loss. Calcite and titanium oxide are known as inorganic single-crystal materials, other than a rock crystal, having a large birefringence; both calcite and titanium oxide have a birefringence larger than that of a rock crystal. However, the rough of calcite is expensive, and the thickness of a half waveplate consisting of calcite becomes as very small as 4 .mu.m because the birefringence of calcite is large, 0.16, at a wavelength of 1.3 .mu.m. Since the hardness of calcite is low (Mohs hardness: 2), it is very difficult to process calcite to have this small thickness. Even if calcite can be thus processed, the product must be handled with enough care. On the other hand, the refractive index of titanium oxide is 2.62 to 2.90, which is largely different from those of silica and other optical waveguide materials. Therefore, when a waveplate consisting of titanium oxide is inserted into an optical waveguide, a loss caused by Fresnel reflection at the end face of the waveguide is large. Consequently, the effect of decreasing the thickness of a waveplate becomes insignificant. For the reasons discussed above, neither calcite nor titanium oxide is a suitable material to be inserted into a lightwave circuit.
In order that a waveguide device in which a half waveplate is inserted be used in practice, the heat resistance and the humidity resistance of the waveplate and the ease in handling the waveplate are also important factors. For example, a waveguide device fabricated on a single substrate is used not only as a single component by itself but also as an "optical and electronic hybrid interconnection" in combination with other lightwave circuits and electric circuits fabricated on the same substrate. The fabrication of these photonic components involves a soldering step performed at about 260.degree. C. and a step performed at a temperature which temporarily exceeds 300.degree. C. Therefore, all the materials used in the fabrication are required to have a heat resistance of about 350.degree. C.
An amorphous polymer plastic material is known as a material which produces a birefringence. Representative examples of such a polymer material are polycarbonate and polyvinyl alcohol. These materials produce an in-plane birefringence when films consisting of the materials are drawn. In practice, large retardation plates for use in liquid-crystal displays are manufactured by using these polymer materials. Retardation plates consisting of polystyrene, a cellulose derivative, polyvinyl chloride, polypropylene, an acrylic polymer, poly(amic acid), polyester, and an ethylene-vinyl acetate copolymer saponified material are also known. However, the polyvinyl alcohol-based material and the cellulose derivative-based material have a low humidity resistance, and the polypropylene-based material is unsatisfactory in toughness. The acryl-based material is difficult to draw because its mechanical strength in the form of a film is low. The polycarbonate-based material is poor in chemical resistance.
The polyvinyl chloride material and the polystyrene-based material are unsatisfactory particularly in heat resistance and are therefore inadequate for the purpose of the present invention. Although the poly(amic acid)-based material and the polyester-based material are considered to have a relatively high heat resistance, none of these materials has a heat resistance of 300.degree. C. or higher which is required for waveguide devices. Also, a waveplate made from any of these organic polymer materials is reduced in birefringence due to activation of molecular motion even at a temperature lower than its softening point (glass transition temperature). This largely degrades the characteristics as a waveplate. In addition, not a few of these organic polymer materials have a saturation water absorption of 2 to 3%. Since, however, water molecules strongly absorb light with optical communication wavelengths to increase the loss, the material to be used as a waveplate must have as low a water absorption as possible.
As discussed above, it is difficult to manufacture waveplates that can be incorporated in optical waveguides by using any of the conventionally known polymer materials.
In summary, the problems of the conventional optical waveplate techniques are as follows. That is, for waveplates using inorganic single-crystal materials, no material having an appropriate birefringence and refractive index by which a waveplate can be incorporated in a waveguide device is available. In addition, these materials are difficult to process and expensive. On the other hand, waveplates consisting of plastic materials have problems in the heat resistance, humidity resistance, and mechanical strength of a material, and in the stability of in-plane birefringence.